1. Field of the Invention
The present invention relates to a thermal head.
2. Description of the Prior Art
FIG. 1 is a perspective view showing a partial section of a thermal head 1 of a typical prior art, and this thermal head 1 comprises a heat resistant substrate 2 made of ceramics or the like, and plural heating resistance elements 3 are linearly disposed on one of its principal planes. The heating resistance elements 3 are selectively heated and driven by an electric constitution which is not shown in the drawing to print on a thermal paper (not shown), for example, in contact therewith. On the other hand, on the opposite side of the heating resistance elements 3 of the heat resistant substrate 2, there is a cooling board 4 made of a material excellent in thermal conductivity such as metal material, and it is adhered to the heat resistant substrate 2 with an adhesive or tacky agent (hereinafter called adhesive) 5 which possesses elasticity at least after curing.
The reason of using such adhesive 5 is as follows. When the thermal head 1 is used at room temperature of, for example, 25.degree. C., the surface of the heat resistant substrate 2 is heated nearly to 80.degree. C., for example, due to heat generation of the heating resistance elements 3. The heat resistant substrate 2 and cooling board 4 differ significantly in the coefficient of thermal expansion. At this time, when both are mutually fixed, the heat resistant substrate 2 and the cooling board 4 are warped in the same principle as a bimetal because of this difference. As a result, although release of heat is smooth at the mutually directly contacting position, but is poor is other positions, and uneven contrast occurs in printing. By using the adhesive 5, besides affixing the heat resistant substrate 2 on the cooling board 4, the difference in thermal expansion between the two is absorbed by the adhesive 5, thereby preventing warping of the heat resistant substrate 2.
In such thermal head 1, in the adhesive 5 satisfying such condition, the thermal conductivity is small, for example, less than 0.5.times.10.sup.-3 cal/cm.sec..degree. C., and the heat from the heating resistance elements 3 is mostly reserved in the heat resistant substrate 2, and when the heating resistance elements 3 are driven at high speed, the printing contrast cannot be controlled due to the residual heat.
In such thermal head 1, it is known to use a cooling compound on the principal plane of the heat resistant substrate 2 instead of the adhesive 5. On the other hand, it is known that waves and warps of height difference of tens to hundreds of microns are formed in the manufacturing process of the heat resistant substrate 2. Accordingly, the layer thickness of the cooling compound 5 between the heat resistant substrate 2 and cooling board 4 differs locally, thereby causing uneven cooling effect. Therefore, when printing thermally, uneven contrast occurs.
FIG. 2 is a perspective view showing a partial section of a thermal head 1a of a second prior art. Relating to this thermal head 1a, the similar and corresponding parts of the foregoing prior art are identified with the same reference numbers. This prior art comprises a heat resistant substrate 2, heating resistance elements 3, and a cooling board 4, which are mutually fixed by bonding in a structure not shown in the drawing. In this example, in the portion of the cooling board 4 confronting the heating resistance elements 3, a cooling compound 6 is placed in a range of width W4 over the entire length in the arrangement direction of the heating resistance elements 3. A pair of long grooves 7 are formed in the cooling board 4 at positions across the cooling compound 6.
The cooling compound 6 is a mixture of fine particles of aluminum oxide or zinc oxide in a size of 1 .mu.m or less and, for example, silicone oil, and it is a greasy matter having viscosity, and its thermal conductivity is generally 1.5 to 3.0.times.10.sup.-3 cal/cm.sec..degree. C., and as compared with the adhesive 5 of the foregoing prior art, this mixture possesses a thermal conductivity of 3 to 6 times higher. After applying the cooling compound 6 between the long grooves 7 on the cooling board 4, when the heat resistant substrate 2 is pressed, the cooling compound 6 is compressed and spread widely on the cooling board 4, and the long grooves 7 are provided to keep this cooling compound 6.
In this prior art, by using the cooling compound 6, the heat generated from the heating resistance elements 3 is easily led out from the heat resistant substrate 2 into the cooling board 4 to be released.
In the thermal head 1a of such prior art, same as in the prior art explained in FIG. 1, warping occurs along the direction vertical to the sheet of paper in FIG. 2 due to the difference in the coefficient of thermal expansion between the heat resistant substrate 2 and cooling board 4. The cooling compound 6 is amorphous, and therefore the heat resistant substrate 2 and cooling board 4 fluctuate in the mutual distance due to such warping along the longitudinal direction of the thermal head 1a. Due to this fluctuation, heat transfer from the heat resistant substrate 2 to the cooling board 4 changes in location, and uneven contrast occurs when thermally printing on a thermal paper.
FIG. 3 is a perspective view showing a partial section of a thermal head 1b in a third prior art, and this example is similar to the foregoing prior arts, and corresponding parts are identified with same reference numbers, and specifically a circuit substrate 8 is mounted on the cooling board 4 aside from the heat resistant substrate 2, and they are connected by bonding wire 9. Between the cooling board 4 and heat resistant substrate 2, there is a cooling compound 6 in a range corresponding to the locations of the heating resistance elements 3, while an adhesive 5 is placed in the remaining region. A step difference 10 is formed in the boundary of the region of the adhesive 5 and the region of cooling compound 6.
In this prior art, using the cooling compound 6, the heat generated by the heating resistance elements 3 is promptly led to the cooling board 4, and by the adhesive 5 formed on the step difference part 10, the heat resistant substrate 2 is affixed to the cooling board 4, and the cooling compound 6 invades into the gap GP by capillary phenomenon, thereby preventing the state of faulty contact at the electric contact part of the circuit substrate 8. Besides, by preliminarily selecting the depth of the step difference 10, the layer thickness of the cooling compound 6 is set.
In the above prior art, the cooling compound 6 and adhesive 5 contact with each other, and in such state, however, the adhesive and the heat resistant substrate 2 and cooling board 4 contacting with the adhesive 5 are wetted with the cooling compound 6, and the adhesive action may not be realized, or be lost suddenly. Besides, the layer thickness of the cooling compound 6 is in a range of scores to hundreds of micrometers, and it is extremely difficult to form the step difference part 10 in the cooling board 4 by cutting at such high precision.
Besides, in this thermal head 1b, in the cooling board 4, the step difference part 10 intervals are formed as protrusions S, and the distance between the protrusions S and the heat resistant substrate 2 is set shorter than the distance from the heat resistant substrate 2 at the step difference part 10, thereby enhancing the heat releasing effect. In such prior art, however, concerning the machining precision of the cooling board 4 for forming the step difference part 10 and protrusions S, dimensional errors of, for example, about .+-.50 micron occur, and the dimensional fluctuations are relatively great, and therefore uneven printing contrast occurs due to reserve of heat on the heat resistant substrate 2.